1. Field of Art
This invention pertains to a method and apparatus by which a metal structural element and a polycarbonate sheet are attached together under torque by means of an attachment strip. It is believed that the invention will find at least a first primary use in air cargo containers wherein polycarbonate sheets are used as the siding or "skin" of the containers and must withstand handling stresses, significant temperature cycling, and, in the event of rapid acceleration or deceleration of the aircraft, shifting cargo which can be thrown under great force against the sides of the container.
2. Prior Art
One of the oldest tasks known to man is how best to transport his possessions from one place to another. From the very first crude bags made of animal hide to the space shuttle, man has been engaged in a continuous attempt to develop means to transport cargo farther, faster, safer, cheaper and easier.
A relative newcomer in this millennia of transportation is the aircraft, and although relatively new, it is now a major player in transporting the property of man. More than any other form of transportation, however, air cargo transport demands that its componentry be not only strong, but lightweight, as additional poundage is more costly with air travel. Additionally, safety has its highest priority in the air, as flight is, even more than ocean travel, intolerant of man's foolishness.
Therefore, the transportation of cargo by air requires, like no other, that the often elusive goals of strength, light weight and safety be accomplished in a single structure. For the transportation of cargo by air, the industry has come to rely almost exclusively on the all-aluminum cargo container, which is first loaded with cargo and is then itself loaded onto the aircraft. This modern air cargo container is a monocoque structure, comprising a rigid frame to which a sheet material, generally referred to as the "skin", is attached to the "bones" of the frame. In these monocoque structures, the skin is load-carrying, sharing the stresses with the frame structure. The loads go from the frame to the skin or from the skin to the frame via their attachment means, which can be rivets, bolts, etc. In construction and at rest, the skin is usually stressed in shear (meaning along the plane of the sheet rather than perpendicular to it), as are the attachment means. At the attachment points the holes in the sheets and frame are formed as close to the diameter of the fasteners as is practical to make the structure as rigid as possible. Clearance between the holes and the fasteners creates "slop" between the parts and therefore reduces the rigidity of the structure as relative movement between the sheet and frame create a "loose," and therefore weak, assembly. The ideal fasteners completely fill the holes in the parts they bring together without "slop", as that creates a structure in which the sheets are stressed in shear when the frames are stressed as a single unit, and is therefore stronger.
In use, however, the air cargo container will also be subjected to hoop tension or stress (i.e., perpendicular to its plane) as when the skins must restrain moving cargo. This is, of course, one of the most important if not the most important function of an air cargo container--to keep its cargo from breaking through its skin and becoming a missile in the event of crash-generating deceleration forces on the aircraft or in the event of turbulence inducing either severe acceleration or deceleration forces. In those flight-threatening events during which accelerations or decelerations occur, the cargo moves against the skins of the container which are thusly stressed in hoop tension which is transferred to the frame, then to the floor locks, then to the floor of the aircraft and eventually to the airplane itself. Hence, the skin material of the container must be able to withstand both significant shear stress and hoop tension.
For obvious reasons, the ideal air cargo container is light in weight, low in cost and capable of withstanding not only the stress encountered inflight, but also the day-today rigors of service--i.e., cargo crates being thrown against the walls, being bumped and jostled--all without being damaged. The best prior art devices used aluminum frames and skins with the sections being riveted together to form a rigid assembly. Rivets were preferably used to eliminate the "slop" between rivet shanks and the holes formed for the rivets, as rivets are "holefilling" (i.e., expand to fill the hole). Containers so made give good useful service, as the structures are rigid, are reasonably light in weight and low in cost.
The main problems these all-aluminum devices encountered in service were with the aluminum skins as they are subject to denting and tearing. Rough use and sharp-cornered boxes take their toll on the skins and often produce tears and dents. When torn, the containers are not serviceable as they are no longer "airworthy" and must be taken out of service and patched before they can be used again. Furthermore, torn skins present a hazard to loading crews and the cargo as the sharp edges cut indiscriminately. The aluminum skins can be made more resistant to such damage by making them thicker and more resistant to tearing, but then weight increases and the cost of flying dead weight (i.e., other than the weight of the transported cargo) makes such use less desirable and eventually not acceptable beyond a certain level. Using higher strength aluminum to solve the problem is actually counterproductive, as the stronger alloys are more brittle and more readily damaged by tearing. Accordingly, there is a need in the art for an improved skin material for air cargo containers.
Polycarbonate sheet has many unique qualities making its use desirable in many industrial applications. It is transparent. It can be struck heavily without being dented, torn or broken. This is because of the material's very low modulus of elasticity; the energy from a potentially damage-inducing blow is absorbed by the sheet without damage as though it were a rubber diaphragm. Hence, polycarbonate plastic sheet would theoretically be an ideal replacement for the aluminum skins. Its transparency would allow the contents of the container to be viewed. It is light in weight, only slightly more costly than the aluminum alloys used and capable of accepting the rough rigors of service without being dented or torn, as it is much more resistant to tearing or denting than is aluminum of comparable thickness and weight. The polycarbonate has substantial drawbacks to its use, however, which until now rendered it not feasible for use as a structural element and certainly not as the skin in a monocoque structure such as an air cargo container.
One such drawback is its very high coefficient of thermal expansion, 0.000037 inches per inch per degree Fahrenheit. This compares to 0.000013 for aluminum or 0.0000063 for steel. If the monocoque structure, the air cargo container for example, must operate in the temperature range of -40.degree. F. to +140.degree. F., as occurs in the air cargo container's service environment (at 30,000 feet versus in the plane's fuselage, on the tarmac, in the hot, desert sun), a typical air cargo-sized panel which is 120 inches between rivet centers when the panel was manufactured at an ambient temperature of 50.degree. F. will be 120.4 inches in length (120 inches.times.90.degree. F. temperature differential.times.0.000037 coefficient) when the temperature is 140.degree. F. and 119.6 inches in length when the temperature is -40.degree. F. In contrast, the distance between rivet centers of the aluminum structure will be 120.14 inches at 140.degree. F. and 119.86 at -40.degree. F. as the coefficient of linear expansion for aluminum is far less. Thus, conventional wisdom has in the past dictated that in order for the polycarbonate sheet to be compatible within this type environment the holes would have to be oversized in diameter (or slotted) by 0.26 inches (120.4-120.14+119.86-119.6) on each side of the panel, allowing for a differential expansion between the polycarbonate sheet and the aluminum frame of 0.52 inches total.
The resultant structure would, however, be at a severe disadvantage compared to its all-aluminum counterpart. The looseness or "slop" of the fasteners in the holes would prevent the sheet and the frame from acting as a load-sharing single unit. Therefore air cargo containers using polycarbonate sheets and conventional attachment means would have to bear the shear loads in the frame alone, which would have to be made larger in order to be stronger, and would therefore be excessively heavy.
Another disadvantage of the polycarbonate which has heretofore prevented its use in air cargo containers is its very low bearing strength, 12,500 psi compared to 100,000 psi for the aluminum alloys used for air cargo container sheets. In other words, the polycarbonate is one-eighth as strong in bearing. To compensate, the polycarbonate skin would have to be attached to the frame at many more locations than is necessary with aluminum skins. This would mean higher costs for the fasteners and the labor for installation, in addition to the heavy, costly frame structures. The resultant structure would be too heavy and costly to compete with the all-aluminum container.
There has heretofore been yet another disadvantage to the polycarbonate's use on air cargo containers; namely, its susceptibility to stress-induced and crazing agent-induced cracking or crazing. When there are residual stresses in polycarbonate, the material is subject to cracking, particularly in the presence of "crazing agents". These include a variety of materials including hydrocarbons, jet fuel cleaning materials, etc., many of which are used near the air cargo containers. A cracked polycarbonate sheet is non-serviceworthy as once cracked, the cracks spread very easily. One crack and the part must be taken out of service. If the residual operational stresses are kept low, for example, under 2000 psi, and the materials are kept free of "crazing agents," the material is relatively free of this incipient cracking problem. As explained above, however, this creates a classic "Catch-22" situation in that an unstressed sheet would require such a heavy frame that the resultant container would be unuseable, whereas riveting the sheet to the frame so that the overall container is unitarily stressed creates a crack-inducing environment, as high stresses are created under the head of the rivet and against the inside of the hole by the expanding rivet shank.
Because of these disadvantages, the use of polycarbonate has heretofore been restricted to applications where it "floats" in its frame, as in signs and aircraft windows, and has not been used as a genuine structural component. For example, in the reference book published by the principal manufacturer of polycarbonate sheets, the means and methods displayed for attaching the sheets specify loosely torqued bolts in oversized holes with a silicone cushion. Certainly, polycarbonate sheet material has not heretofore proven to be an acceptable substitute for the aluminum "skin" on a monocoque airline cargo structure because no acceptable means for attaching the polycarbonate to the aluminum frame was known. Accordingly, there has existed a need in the art for a means for rigidly attaching polycarbonate sheet material to a metal structural element in a way to allow the polycarbonate to act as a structural component, while at the same eliminating or substantially alleviating the material's tendency to crack or craze under stress.